This invention relates to machines for forming, filling and sealing packages from a thin flat strip of packaging material, the strip of packaging material being formed to a tubular configuration, sealed longitudinally at overlapped edge portions, intermittently sealed transversely along lines spaced longitudinally of the tube, and filled with measured quantities of product between successive transverse sealing operations. More particularly, the invention relates to machines in this general category sometimes characterized as being of the "tubeless" type. In a tubeless form, fill and seal packaging machine the formation of the tube of packaging material does not occur on or about a supporting tube but instead the space within the formed tube is maintained as free as possible of obstruction, a narrow elongated sealing back-up bar and one or more narrow elongated tube spreaders being the only impediments to free product fall within the formed tube. Such machines have been generally satisfactory in the past but difficulties have been encountered in the longitudinal edge sealing operation. With a tubeless machine the narrow elongated sealing back-up bar may tend to flex or otherwise undergo slight movement when subjected to the considerable force exerted by its complementary sealing bar. Resulting loss of parallelism and non-uniform pressure distribution between and throughout the length of the two bars may of course have serious detrimental effects in the sealing operation and partially open or otherwise inferior longitudinal edge seals may result. Alternatively, strengthening and increasing the mass of the back-up bar creates further obstruction to free product fall and tends to defeat the very purpose of the tubeless machine.